In the recent development of semiconductor fabrication technology, the continuous miniaturization in IC devices demands more stringent requirements in the fabrication environment and contamination control. When the feature size was in the 2 .mu.m range, a cleanliness class of 100.about.1000 (which indicates the number of particles at sizes larger than 0.5 .mu.m per cubic foot of air) was sufficient. However, when the feature size is reduced to 0.25 .mu.m, a cleanliness class of 0.1 is required. It was recognized that an inert mini-environment may be the only solution to future fabrication technologies when the device size is further reduced. In order to eliminate micro-contamination and to reduce native oxide growth on silicon surfaces, the wafer processing and the loading/unloading procedures of a process tool must be enclosed in an extremely high cleanliness mini-environment that is constantly flushed with ultrapure nitrogen that contains no oxygen and moisture.
Different approaches in modern clean room design have been pursued in recent years with the advent of the ULSI technology. One approach is the utilization of a tunnel concept in which a corridor separates the process area from the service area in order to achieve a higher level of air cleanliness. Under the concept, the majority of equipment maintenance functions are conducted in low-classified service areas, while the wafers are handled and processed in more costly high-classified process tunnels. For instance, in a process for 16M and 64M DRAM products, the requirement of contamination control in a process environment is so stringent that the control of the enclosure of the process environment for each process tool must be considered.
The stringent requirement creates a new mini-environment concept which is shown in FIG. 1. Within the enclosure of the mini-environment of a process tool 10, an extremely high cleanliness class of 0.1 (which means the number of particles at sizes larger than 0.1 .mu.m per cubic foot of air) is maintained, when compared to a cleanliness class of 1000 for the overall production clean room area 12. In order to maintain the high cleanliness class inside the process tool 10, the loading and unloading sections 14 of the process tool must be handled automatically by an input/output device such as a SMIF (standard mechanical interfaces) apparatus. A cassette of wafer can be transported into the process tool 10 by a SMIF pod 18 situated on top of the SMIF apparatus
In a conventional SMIF apparatus 20 such as that shown in FIG. 1, the apparatus 20 consists of a robotic arm or a rotating arm 32 (shown in FIGS. 2A and 2B) which is normally configured for gripping the top of a cassette 30 from a platform on which the cassette 30 is placed (inside a pod). The rotating arm 32, which is equipped with a gripper assembly 40 at one end, is capable of transporting the cassette 30 into the process tool and place it onto a platform 16 vertically such that the cassette 30 is oriented horizontally. At the beginning of the process, an operator positions a SMF pod 18 on top of a platform/elevator 22 which contains a cassette 30 holding, for instance, 24 wafers in an upright position. The elevator then descends into the SMIF apparatus 20 for the rotating arm 32 to transport the cassette 30 into the process tool. The SMIF system 20 therefore utilizes clean isolation technology to maintain a high class clean room effectiveness near wafers and processing equipment. The operation of the robotic arm 32 or the gripper assembly 40 is controlled by an ancillary computer (not shown) of the process tool 10. The cassette 30 carries wafers or other substrates that are being processed.
The SMIF apparatus 20 has a port (or opening) 24 which is intimately mated with an opening 26 in the sidewall 28 of the process tool 10. The SMIF pod 18 is a sealed container with an opening at the bottom and is capable of preventing contamination to the cassette held therein. The pod may also be equipped by a tagging system for the automated identification and recognition of the parts contained in the pod to prevent mis-processing, of the wafers and to track through the host computer of the product-lot serial numbers. The tagging system is mounted on the pod, while a probe assembly is mounted on the port of the SMIF apparatus 20. The SMIF apparatus 20 is therefore an effective interface between an operator and the process tool 10 in that the transporting of cassette can be conducted in a completely automated fashion to avoid human contact by the operator. This insures that the cassette 30 is transported through a highly clean environment into the process tool 10.
Occasionally, the SMIF apparatus 20 or the process tool 10 requires repair or maintenance procedures to be performed on them. When one of such needs arises, the SMIF apparatus 20 must be disassembled from the process tool 10 and be physically pushed away in order to provide access to an operator for performing the repair or maintenance.
A rear view and a side view of the SMIF apparatus 20 is shown in FIGS. 2A and 2B, respectively. The robotic arm or rotating arm 32 is mounted to the SMIF apparatus 20 through an aperture 34 (shown in FIG. 3A) at the lower end 36. The upper end 38 is equipped with a gripper assembly 40. The gripper assembly 40 is operated by a chain drive 42 for tilting of the gripper assembly 40 when holding a wafer cassette 30. The motion of the gripper assembly 40 of being raised up or down is controlled by the rotating arm 32. It should be noted that in FIG. 3A, only the chain drive 42 is shown for the rotating arm 32 for simplicity reasons.
The rotating motion of the rotating arm 32 is controlled by a worm gear 46 and a worm 48. The worm gear 46 is rigidly fixed to a shaft 50 mounted through the aperture 34 at the lower end 36 of the rotating arm 32. To control the motion of the rotating arm 32, an upward-tilt sensor arm 52 and opto-electrical sensor 54 are used to control the extreme upward tilt position. The extreme downward tilt position is controlled by the downward-tilt sensor arm 56 and the opto-electrical sensor 58. The sensor arms 52, 56 are mounted coaxially with the rotating arm 32 by frictional engagement to the shaft 50. A detailed view of the worm gear 46 and the worm 48, and the sensor arms 52 and 56 are shown in FIG. 3B.
The operation of the tilt control device for the gripper assembly 40 can be explained in FIG. 3B. It should be noted that the view presented in FIG. 3B is of a mirror image of that shown in FIG. 3A since the view in FIG. 3B is taken from a direction opposite to that taken for FIG. 3A. The positioning of the sensor arms 52, 56 and the opto-electrical sensors 54, 58 are therefore reversed in the two Figures. During a normal operation, the gripper assembly 40 and the wafer cassette 30 swings upwardly to properly seat all the wafers (not shown) in the cassette 30 so that the cassette can be later fitted into a pod without breaking the wafers that may be improperly seated and therefore protruding from the cassette top. Once the wafers are properly seated into the wafer cassette 30, the gripper assembly 40 tilt downwardly to a normal horizontal position. The extreme downward-tilt position is controlled by the up-tilt sensor arm 56 and the opto-electrical sensor 58, while the extreme upward-tilt position is controlled by the downward-tilt sensor arm 52 and the opto-electrical sensor 54. When the opto-electrical sensor 58 is triggered by the sensor arm 56, it is an indication that the extreme downward-tilt position has been reached. Similarly, when the opto-electrical sensor 54 is triggered by the sensor arm 52, it is an indication that the extreme position of the upward-tilt of the gripper assembly 40 has been reached.
Under normal operation conditions, the extreme downward-tilt position is the position where the gripper assembly 40 is returned to a horizontal position. However, it has been observed that an over downward-tilt motion of the gripper assembly 40 can occur as a result of a failure of the downward-tilt sensor arm 56 to trigger the downward-tilt opto-electrical sensor 58. This failure of the sensor mechanism to stop the gripper assembly 40 from over-tilting creates serious problems. The wafers contained in the cassette 30 may slide out of the cassette when it is positioned in an over-tilt position and cause breakage of the wafers. The over-tilt problem may be caused by several reasons. First, since the sensor arm 56 is attached to the shaft 50 of the worm gear 46 by frictional engagement, the sensor arm 56 may become loose on the shaft and shift away from its correct position. The sensor arm 56 may have been mounted in a wrong position after a maintenance procedure has been performed on the sensor mechanism and thus causing a malfunction between the sensor arm 56 and the opto-electrical sensor 58. Secondly, the existence of any bugs in the control program for the rotating arm may cause malfunction of the arm during a downward-tilt motion. Thirdly, the opto-electrical sensor 58 may not function properly and thus fails to stop the downward-tilt motion of the rotating arm.
It is therefore an object of the present invention to provide an apparatus for preventing the over-tilt of a gripper assembly in a cassette loader that does not have the drawbacks and shortcomings of the conventional over-tilt protection devices.
It is another object of the present invention to provide an apparatus for preventing the over-tilt of a gripper assembly in a cassette loader that does not require the alteration of the control programming for the cassette loader.
It is a further object of the present invention to provide an apparatus for preventing the over-tilt of a gripper assembly in a cassette loader by the addition of an independent sensor system for preventing the occurrence of over-tilt of the rotating arm.
It is another further object of the present invention to provide an apparatus for preventing over-tilt of a gripper assembly in a cassette loader by providing a mechanical stop which functions to prevent over-tilt even when the additional apparatus fails to detect the over-tilt.
It is still another object of the present invention to provide an apparatus for preventing the over-tilt of a gripper assembly in a cassette loader by utilizing an additional sensor system that is permanently affixed to the rotating arm shaft that would not come loose on the shaft.
It is yet another object of the present invention to provide an apparatus for preventing the over-tilt of a gripper assembly in a cassette loader by providing an apparatus that can be installed and adjusted easily on the rotating arm assembly.
It is still another further object of the present invention to provide an apparatus for preventing the over-tilt of a gripper assembly in a cassette loader that will stop the motion of the loader instantaneously when the additional sensor system is triggered.
It is yet another further object of the present invention to provide an apparatus for preventing the over-tilt of a gripper assembly in a cassette loader that also provides a warning signal to indicate to a machine operator the existence of an over-tilt condition.
It is still another further object of the present invention to provide an apparatus for preventing an over-tilt of a gripper assembly in a cassette loader that does not require an excessive modification of the rotating arm assembly.
It is yet another further object of the present invention to provide an apparatus for preventing the over-tilt of an gripper assembly in a cassette loader that also provides a manual override device such that a machine operator may temporarily override the power-down condition of the gripper assembly and thus returning the gripper to a safe position without disassembling the rotating arm assembly.